This invention relates generally to an improved device for use in depositing condensation coatings on various substrates. More particularly, the invention relates to an improved modular deposition chamber for depositing para-xylylene polymers on electrical component parts and the like.
Para-xylylene polymers are employed as coatings for various electronic components due to their desirable physical and electrical properties. One advantage of poly-para-xylylene coatings is that extremely thin layers of such coatings are capable of exhibiting highly desirable physical and electrical properties. Because para-xylylene coatings are applied in very thin layers, heat tends to dissapate rapidly from the underlying components. Thus, the coated components cool down quickly and are less prone to temperature related degredation than similar components bearing other types of coatings.
In further contrast to conventional polymer coatings, para-xylylenes are generally not prepolymerized prior to application on the coatable substrate. This is so because the para-xylylene polymers are not given to simple extrusion, melting or molding as are many of the conventional thermoplastics. Additionally, because the para-xylylenes are generally insoluble in commonly used organic solvents, it is impractical to employ traditional solvent deposition techniques for applying poly-para-xylylene coatings.
Accordingly, in most commercial applications, para-xylylene polymers are deposited on desired substrates by a pyrolytic deposition process known specifically as the xe2x80x9cparylene process.xe2x80x9d Such process begins with the vaporization of a cyclic di-para-xylylene dimer. The dimer is pyrolytically cleaved at temperatures of about 400 to 750xc2x0 C. to form a reactive para-xylylene monomer vapor. Thereafter, the reactive monomer vapor is transferred to a deposition chamber wherein the desired substrates are located. Within the deposition chamber, the reactive monomer vapor condenses upon the desired substrates to form a para-xylylene polymer or co-polymer film.
Any monomer vapor which fails to condense within the deposition chamber is subsequently removed by a cold trap which is maintained at approximately xe2x88x9270xc2x0 C.
The entire parylene process is generally carried out in a closed system under constant negative pressure. Such closed system may incorporate separate chambers for the (a) vaporization, (b) pyrolysis, and (c) deposition steps of the process, with such chambers being connected by way of appropriate plumbing or tubular connections.
A primary consideration in the parylene deposition process is the achievement of uniform coating thickness on the desired substrates. Unlike conventional polymer coating systems, the condensation deposition of parylene coatings is capable of depositing even ultrathin films without running or uneven areas resulting upon the substrates, provided that the monomer vapor is homogeneously and evenly distributed on the surface of the substrate. Thus, the design and functioning of the deposition chamber is critical to the achievment of uniform vapor distribution with resultant even coating deposition.
Another important consideration in the parylene deposition process is the minimization of waste. Currently, parylene raw materials may cost as much as $400 to $600 per pound. Thus, there exists substantial economic motivation to preserve and conserve the parylene materials during the coating process. One particular area in which a great deal of material is wasted is on the various internal structures of the prior art parylene deposition chambers. It must be appreciated that the condensation deposition of coatings is not substrate selectivexe2x80x94i.e. the vapors have no way of seeking out only the desired substrate. Instead, the monomer vapor will condense and polymerize on any reduced temperature object with which it comes in contact. As a result, the entire inner surface, of the chamber, and all of the objects positioned therein will become covered with the parylene coating. Thus, the interior of the chamber and any existing hardware must be cleaned periodically to remove wasted parylene polymer.
The parylene deposition chambers employed in the prior art have generally provided less than optimal coating uniformity due to inferior distribution and homogeneity of the vapor within the deposition chamber. Also, because of the particular chamber design, the prior art deposition chambers are associated with a great deal of waste of the parylene chemicals.
At least one deposition chamber of the prior art employs a system of baffles, positioned adjacent a monomer vapor inlet line, so as to disperse the flow of vapor as in enters the deposition chamber. Such baffles are intended to uniformly distribute the monomer vapor throughout the interior of the deposition chamber thereby insuring uniform coating thickness on the desired substrates. In practical application, however, the various baffle designs employed in the prior art devices have failed to provide truly optimal vapor distribution within the chamber. As a result, less than optimal coating uniformity has been realized. Additionally, the presence of such baffles occupies otherwise useable space within the chamber and results in greater surface area within the chamber. Such increased surface area, accordingly, increases the amount of parylene waste due to the nonselective deposition of the polymer on the baffles as well as on the desired substrates.
Also, the deposition chambers of the prior art incorporate substrate holding racks which are supported only by one or more members extending from the bottoms thereof. The absence of any support member fixing the top end of such holding rack to the surrounding deposition chamber structures results in a rather unstable arrangement. Specifically, when relatively heavy parts are unevenly distributed on the upper shelves of the holding rack such rack may tend to lean against the surrounding baffles or deposition chamber walls.
The present invention overcomes the above described problems of the prior art, and others, by providing a condensation coating deposition chamber wherein uniformity of monomer vapor is maintained by inducting a rotary flow pattern within the chamber. Such rotary flow pattern obviates the need for baffles or other hardware elements thereby lessening the amount of polymer wasted during the process. Additionally, the parylene deposition chamber of the present invention provides greater versatility than the prior art devices because it is of modular design and, thus, easily detachable from the pyrolytic generating unit. Also, the deposition chamber of the present invention incorporates a substrate support or holding rack which is pivotally supported at its top end as well as its bottom end.
In accordance with the present invention, there is provided a condensation coating deposition chamber comprising a tank-like chamber body having a floor, a cylindrical wall, and an openable and closable lid. The monomer vapor enters tangentially near the top of the chamber through a tangentially connected monomer supply line. Such tangential entry of the monomer vapors results in a generally annular rotational flow of the vapors as they descend axially through the inner confines of the deposition chamber. Further in accordance with the invention a substrate support fixture is positioned centrally within the deposition chamber. As the flow of monomer vapors descends within the inner confines of the deposition chamber, the support fixture is rotated, preferrably in a direction opposite the rotational flow of the entering vapors. Also, the fixture is specifically sized such that an annular space exists between the outer edges of the rack and the inner wall of the chamber. The provision of such annular space provides for an even flow of vapor around the fixture.
Further, in accordance with the invention, the substrate support fixture may comprise a multi-tiered rack having a plurality of substrate support shelves positioned horizontally therewithin. Each such substrate support shelve is provided with a multiplicity of perforations through which the monomer vapor may flow. Such perforations further enhance the degree of permeation and evenness of the vapor flow within the deposition chamber. The substrate support fixture is preferably pivotally connectable to the lid of the deposition chamber as well as the floor of the chamber, thereby providing uniform top to bottom support for the substrate support fixture so as to prevent lateral movement or shifting thereof and avoiding any resultant contact between the edges of the support fixture with the surrounding deposition chamber wall.
In accordance with an even further aspect of the invention, the deposition chamber of the present invention may be of modular design so as to be easily detachable from the attendant pyrolytic generation equipment. Such modular design permits the deposition chamber to be moved to a separate area for loading/unloading, cleaning and maintenance. Also, upon detachment of one modular chamber, another modular chamber may be immediately substituted in its place. Such flexibility provides for optimal utilization of a single pyrolytic vapor generating unit with multiple interchangable deposition chambers. Additionally, to facilitate movement of the modular sections, both the pyrolytic generator and deposition chamber modules are caster mounted to permit rapid and easy relocation as desired.
A principal object of the invention is to provide a condensation coating deposition chamber wherein improved vapor flow characteristics and design will result in uniform and even coating deposition, even at ultrathin film thicknesses.
Yet another object of the invention is to provide a condensation coating deposition chamber which will prevent waste of chemicals by avoiding the need for certain space occupying objects, such as baffles, which increase the surface area within the chamber.
Yet another object of the invention is to provide a condensation coating deposition chamber which is of modular design so as to be easily interchangeable and detachable from the attendant vapor generating unit.
A still further object of the invention, is to provide a substrate support fixture which is capable of being rotated within the deposition chamber so as to subject each substrate to uniform vapor concentrations and conditions.